Acrylic acid is an important raw material in the plastics industry since it can be processed further to form polyacrylates and copolymers. Acrolein serves as starting material in the industrial synthesis of acrylic acid. An important industrial process for preparing acrolein is the partial oxidation of glycerol. This is obtained in large amounts in the cleavage or transesterification of glycerides.
In such glyceride cleavage processes or glyceride transesterification processes, an about 70-90% strength, often from 75 to 85% strength but usually about 80% strength, crude glycerol which, depending on the production process used, contains further components is obtained. The greatest secondary constituent is often from about 1% to about 20% of water. In addition, sodium and potassium salts of various mineral acids, for example salts of hydrochloric, sulfuric or phosphoric acid, are generally present. In some cases, salts of organic acids, for example of citric acid, are also present. In addition, free fatty acids and soaps are present in the crude glycerol. The undesirable organic constituents are referred to as MONG (for “matter organic non-glycerol”).
When the crude glycerol produced in this way is used in subsequent synthesis processes on an industrial scale, the salts, in particular chlorides, cause corrosion problems. They therefore have to be removed from the crude glycerol. In the subsequent synthesis of acrolein, the organic constituents of the crude glycerol can lead to undesirable by-products and to problems in purification. In addition, they can, like the salts, have an undesirable effect on catalysts and lead to carbon deposits.
One process for preparing acrolein from glycerol and for further processing to give acrylic acid is known from WO-A-2006/092272. Here, the glycerol is obtained by cleavage of triglycerides and is processed further directly to give acrolein. The separation of acrolein from the reaction mixture is effected by means of distillation. In this process, even high-boiling constituents are removed. A remaining mixture, which contains high proportions of water and glycerol, is recirculated to the original reaction mixture for the synthesis of acrolein.
A further process is described in DE-A-10 2005 028 624. Here, acrolein is prepared from an aqueous glycerol phase. The acrolein is depleted from the resulting reaction product. The depleted reaction phase is recirculated in a circuit to the reaction region, so that unreacted glycerol can be converted into acrolein.
The processes of WO-A-2006/092272 and DE-A-10 2005 028 624 have the advantage that, because of the circuit via which the unreacted glycerol is returned to the synthesis, an overall higher proportion of the glycerol used is reacted.
However, a disadvantage is that in these processes part of the undesirable by-products, in particular the salts and the chloride ions, interfere in large-scale industrial use over the long term and lead to increased downtimes. WO-A-2006/092272 therefore proposes purifying the mixture comprising water and glycerol from the high boiler separator by means of a membrane. However, this only partly alleviates the disadvantage described. Many constituents, in particular the salts and the chloride ions, cannot be removed completely, if at all, by means of such a membrane. This can result in the problem that undesirable constituents increasingly accumulate in the circuit when the reaction is carried out for a prolonged time and these adversely affect the reaction or damage parts of the apparatus by corrosion.